Two groups of horses were each fed either a control diet of ground hay and concentrates (4 horses), or a Similar diet with 10% added fat after undergoing a baseline Standard Exercise Test (SET). The SET was a stepwise, incremental test to exhaustion on an equine treadmill set at a 6% slope. Resting and working heartrates and rectal temperatures were monitored, and venous blood was collected at rest, and every 3 minutes during exercise, just prior to each speed change. Blood was analyzed for pH, hemoglobin, and pCO₂, and base excess and plasma bicarbonate levels were calculated using nomogram equations. Plasma samples were analyzed for albumin at each step, and for sodium, potassium, chloride, and lactate at rest and exhaustion only. The plasma SID was calculated at rest and exhaustion by the following equation: ([Na⁺] + [K⁺]) - ([Cl⁻] + [Lactate]) The SET was performed after 16 days of interval training, and once more after another 16 days of interval training. Differences over time during exercise were found: heartrate, lactate, and potassium increased (p<.001), and hemoglobin increased (P<.01). Decreases were found in PH, pCO₂, bicarbonate concentration, base excess, and chloride (p<.001). Training effects were found in resting and working heartrates, pCO₂, bicarbonate concentration, and base excess, which all decreased during exercise with training. Hemoglobin increased during exercise with training. There were treatment * SET interactions for Strong Ion Difference, base excess, lactate concentration, pCO₂, and pH. There were no differences found between groups for any of the variables measured. Both groups showed improvements in fitness with training, and the fat group had a higher level of plasma lactate by SET 3. These results suggest that a high fat diet combined with interval training may have some effects on plasma lactate, and that training alone can affect many variables. The results also give evidence to support the evaluation of SID during exercise in horses. / Master of Science